1. Field of the Invention
This invention relates generally to gemstone polishing processes and an apparatus for effecting such methods, and more particularly to methods for effecting ultra-polishing, cleaning, planarization and/or deposition of angstrom level coatings on natural/synthetic single or polycrystalline diamonds and gemstones, chemical vapor deposited diamonds, precious metals and other durable materials for the purpose of producing a final surface finish at sub-nanometer level of roughness and modification of surface properties. The ultra-polish technique described in this invention may also be applied to metallographic and semiconductor device sample preparation to enable the microscopic examination of surfaces for characterization of structure and chemical properties.
2. Description of the Prior Art
The high cost of gemstones, and particularly diamonds, owes to their rarity in nature, their difficulty in mining, and their cost for finishing. Currently accepted techniques for planarizing and polishing gem quality diamonds, for instance, utilize a grinding and polishing tool that is somewhat inefficient, with the final polishing result heavily dependant on the skill of the operator using the grinding tool. In addition, the surface planarity and surface polish quality is dependant on the diamond powder size and the properties of the grinding wheel.
Polishing is often necessary as a post-processing step after the raw gemstone is cut to form the finished stone. In the gem trade as well as in the manufacturing of industrial diamond products, cleavage has more or less been replaced by sawing. Sawing is carried out by pressing a single diamond crystal against a thin, rotating (˜104 turns/minute) phosphor-bronze wheel with an edge impregnated by a mixture of diamond powder and oil. In general sawing is carried out along the {110} and {100} planes. Sawn diamond surfaces show numerous sawing grooves that need subsequent polishing.
Under conventional processes, diamond is polished by pressing a crystal against the surface of a rotating cast-iron wheel, or scaife, charged with a mixture of diamond powder and oil. The scaife rotates at high speed (50 m/s) with the diamond facet to be polished being pushed against the scaife under 10-25 kg/mm2 pressure. This method has been used for planarizing and polishing gem quality diamonds; however, its processing efficiency is extremely low and the ability to achieve a desired surface finish is limited.
Another characteristic of processing using a scaife is that the rate of material removed using diamond polishing varies strongly with crystal orientation. Such a characteristic has contributed to the complexity of achieving a uniform high quality of polish across all crystal planes. For instance, the polishing rate for {110} and {100} faces along the ‘soft’ directions <100> is two to three orders of magnitude higher than the ‘hard’ directions <110>. The {111} plane is abrasion resistant in all directions. However, the use of the diamond powder as an abrasive leads to material removal on {111} planes by micro-fracture at a very slow polishing rate. Polishing along the ‘soft’ directions is thought to proceed via the transformation of sp3 hybridized carbon (diamond) to a less dense form of carbon (sp2 hybridized) which detaches itself from the diamond. Scanning tunneling microscope (STM) topography shows that polished diamond surfaces (polished along ‘soft’ directions) are rough on a nanometer scale, and are covered by grooves, 20 to 100 nm wide and 4 to 12 nm deep. By lateral movement of the diamond during polishing, the roughness of polished facets on single crystals achieved by mechanical polishing can be as low as 1 nm for {110} plane and 2 nm for {100} plane. Polishing in the hard <111> direction occurs by fracture and chipping but no cleaving in the nano-scale range, thus resulting roughness of the surface obtained is significantly higher. Achieving this level of polish finish requires extreme skill, and is very time consuming in addition to imposing repeated re-conditioning of the scaife. Thus, polishing a gem quality diamond requires such great skill where polishing is carried out while examining the crystallographic planes and orientation to locate the plane to be possibly polished. This has led to making diamond polishing complicated and expensive.
Other drawbacks to current polishing techniques involve the grinding material itself. As described above, a diamond is so hard a material that it is generally believed that there is no substitute for it; therefore, it is only natural to consider that there is no abrasive for diamond except diamond itself (i.e., grinding and polishing using diamond). Thus, grinders have been devised for polishing diamonds in which a diamond abrasive embedded in different kinds of binders are used for grinding and polishing. Examples of such grinders include a resin bonded diamond wheel utilizing phenol resin, a metal bonded diamond wheel, a vitrified bonded diamond wheel utilizing feldspar/quartz, and an electroplated diamond grinding wheel.
Polishing problems are not limited to diamonds. The methods used for sample preparation of metallographic sections, semiconductor device sections and other materials are typically a combination of one or more techniques involving mechanical grinding and polishing using a variety of abrasive media and/or focused ion beam polishing. Mechanical methods are time consuming although there have been major advances in automation and control. Focused ion beam polishing makes it possible to polish small regions with the ability to precisely reach the plane of interest. But surface and sub-surface damage due to implantation and use of energetic monomer ions is inevitable.
Another technology for treating surfaces is using energetic-ion sputtering for etching and thinning in manufacturing and depth-profiling in analytic instruments. However, energetic-ion sputtering causes subsurface damage and accumulated roughness because energetic-ion sputtering uses monomer ions. Individual monomer atoms or molecules have energies on the order of thousands of electron volts that can cause residual surface damage to the material being polished.
Accordingly, the need remains for an improved method for polishing surfaces such as diamonds, gemstones, and other materials that overcomes some of the drawbacks that exist in the prior art.